1. Field of the Invention
This invention relates to a laser recording apparatus and a laser irradiating apparatus for performing optical scanning on an object, such as a silver-salt film and the like, for example, using a light output of a semiconductor laser, to record a half-tone image.
2. Description of the Related Art
Heretofore, in an image recording apparatus used, for example, in medical treatment for recording a half-tone image on a silver-salt film and the like, a gas laser, such as a He-Ne laser and the like, has been used for a laser light source. Since a light output of a gas laser can not be directly modulated, however, it is necessary to provide a separate light modulator, such as an acoustooptic modulator (AOM) and the like. Recently, however, an inexpensive semiconductor laser (a laser diode) has found widespread use, primarily because the gas laser and the AOM are expensive.
The semiconductor laser has the advantage that its light output can be directly modulated by modulating its driving current, and a separate light modulator such as the AOM is not required. However, the limited dynamic range of the semiconductor laser causes its own problems.
FIG. 4 illustrates the relationship between the driving current and the light output of the semiconductor laser. The semiconductor laser produces an LED emission without producing a laser emission when the driving current is from zero up to the threshosld current I.sub.TH, and finally produces a laser emission when the driving current exceeds I.sub.TH. The relationship between the driving current and the light output of the semiconductor laser shows a nearly linear characteristic at each of the LED emission area and the laser emission area, but the slope of each region is very different from each other. The relationship also has a sharp inflection point at I.sub.TH. The ratio of P.sub.MAX to P.sub.TH is at most between 10 and 100 to 1, where P.sub.TH is the light output at I.sub.TH. Hence, there is the problem that, when an image is recorded on a silver-salt film when the semiconductor laser is operating in the laser emission area, that is, from P.sub.TH to P.sub.MAX, a desired density range sometimes can not be obtained.
This fact will be explained with reference to FIG. 5(A). The first quadrant I of FIG. 5(A) shows the input characteristic of a recorded image, where the abscissa represents the input signal I and the ordinate represents the recorded density D. In this case, it is assumed that the driving current of the semiconductor laser is proportional to the input signal. Hence, the input signal is equivalent to the driving current. The driving current, that is, the input signal, is converted into a light output P by the characteristic of the semiconductor laser shown in the fourth quadrant IV, where the ordinate represents the light output P. The amount of exposure E is a product of the light output P and an exposure time T, and becomes ##EQU1## if the exposure time T has a constant value T.sub.O. Hence, E and P are equivalent to each other.
The amount of exposure E is converted into the recorded density by the relationship between the amount of exposure and the recorded density, the so-called HD curve shown in the second quadrant II. Since density is defined by the logarithm having the base 10 of transmittance or reflectivity, the amount of exposure E is also usually expressed by the logarithm having the base 10. Accordingly, a logarithmic transformation having the base 10 is performed at the third quadrant III. If the abscissa of the second quadrant is represented by a normal scale, rather than by logarithm, the diagram of the logarithmic transformation in the third quadrant becomes unnecessary.
As is apparent from the above-described explanation, since no operation is performed relative to the signal, the characteristic shown in the first quadrant becomes a composite characteristic itself of the characteristic of the semiconductor laser and the characteristic of the silver-salt film.
A graph such as that shown in FIG. 5(A) may be used to determine the image density D that will result from a given input I. To use the graph, an input I is selected and a resulting laser power P is determined in quadrant IV. Since the power P is equivalent to the amount of exposure E, the logarithm to base 10 is next determined in quadrant III. Finally, the image density D is determined from the HD curve in quadrant II.
By repeating this cycle through quadrants IV, III and II for a number of different input powers I, the composite characteristic shown in quadrant I, which directly shows the input density D as a function of input power I, may be constructed. For example, for input power I.sub.TH, quadrant III gives the logarithm to base 10 of exposure as shown by the solid line. Using curve (b) in quadrant II gives the density for this exposure as D.sub.F. Thus, reverting back to quadrant I, a point may be placed at the intersection of both D.sub.F and I.sub.TH to indicate directly that for input power I.sub.TH, a density D.sub.F will result when curve (b) in quadrant II is used.
If it is assumed that the HD curve of the silver-salt film has a characteristic having a very narrow latitude as shown by (a) in the second quadrant, recording can be performed from the base density D.sub.B of the silver-salt film at a desired laser emission area up to the maximum density D.sub.MAX, as shown by (a) in the first quadrant. However, since the latitude of a normal silver-salt film is at least between 100 and 1000 to 1, the characteristic as shown by (b) in the second quadrant is obtained. In this case, the recorded characteristic becomes as shown by (b) in the first quadrant, and if it is assumed that the driving current of the semiconductor laser is used only down to I.sub.TH, a fog having a density of D.sub.F is produced. The curve (b) is for the case of adjusting to the maximum density D.sub.MAX. If the amount of exposure is decreased using a low-power semiconductor laser or an optical filter so that a fog is not produced, the curve (b) in the second quadrant is shifted to the left to become the curve (c). In this case, although a fog is not produced, the maximum density obtained decreases from D.sub.MAX to D.sub.M, and the desired density range can not be obtained.
As a conventional example for dealing with the above-described disadvantage, there is Japanese Patent Public Disclosure (Kokai) No. 63-102552 (1988). This is a method in which the total area of the LED emission area and the laser emission area of a semiconductor laser is used, and a look-up table is used for correcting its nonlinear property, like general nonlinear correction.
A characteristic obtained by this method is shown in FIG. 5(B). The first quadrant in FIG. 5(B) corresponds to the first quadrant in FIG. 5(A). In this case, however, the abscissa represents the input signal N, because the input signal is converted into driving current after having been corrected by the look-up table. The second quadrant shows a composit characteristic of the characteristic of the semiconductor laser and the characteristic of a silver-salt film, and is identical to that shown by (b) in the first quadrant in FIG. 5(A). The third quadrant shows a correction characteristic. In this example, it becomes a characteristic which is symmetrical to that of the second quadrant, because the first quadrant has a linear characteristic. This transformation process is described in detail, for example, in Japanese Patent Public Disclosure (Kokai) No. 61-81075 (1986), and a detailed explanation thereof will be omitted.
It is very difficult and therefore impractical to perform the correction in the third quadrant by an analog circuit, and a look-up table of a digital circuit is generally used. Since the output of the look-up table is proportional to the driving current of the semiconductor laser, they are equivalent to each other, and transformation thereof is omitted. In the present conventional example, since the driving current of the semiconductor laser is used from zero to I.sub.MAX and the light output can be used from zero to P.sub.MAX, it is possible to scan a silver-salt film which has a large latitude. In this example, however, since the non-linear characteristic to be corrected has a steep slope and a sharp inflection point, and is corrected by the look-up table of the digital circuit, there occur various inconveniences. An amount less than 1 bit can not be expressed by the digital circuit, and the density difference which can be expressed is limited by the output of the look-up table, that is, 1 LSB on the I axis and becomes .DELTA.D, as shown in FIG. 5(B). .DELTA.D is proportional to the slope of the characteristic of the second quadrant. When .DELTA.D is large, a so-called pseudocontour appears in a recorded image. In order to suppress .DELTA.D to a small value, the number of bits of the look-up table must be large. Furthermore, as described above, there is a sharp inflection point in the characteristic, and gradation distortion appears in a recorded image if the the inflection point is exactly corrected. However, it is actually difficult to exactly correct the sharp inflection point.
As another conventional example, there is Japanese Patent Public Disclosure (Kokai) No. 61-124921 (1986) (U.S. Pat. No. 4,679,057). This is a method in which a limited number of light outputs are utilized within the laser emission area of a semiconductor laser and the amount of exposure is changed by performing pulse-width modulation of the light outputs to broaden dynamic range, somewhat like a multivalued dither method. This method is based on the reciprocity law that density is determined by the amount of exposure which is the product of light intensity and exposure time. That is, in the above-described formula (1), density is constant if E is constant, whether P is changed while keeping T constant, or T is changed while keeping P constant. Accordingly, in the present conventional example, the dynamic range of the amount of exposure becomes large while leaving the dynamic range of the light output of the semiconductor laser narrow, and the relationship between the input signal and the amount of exposure can also have a nearly linear characteristic. This example has, however, the following problems.
FIG. 7 shows distributions of the amount of exposure when optical scanning is performed with turning on and off a laser light. In FIGS. 7(A), 7(B) and 7(C), the symbol a indicates the intensity distribution of the laser light, which nearly shows a Gaussian distribution. The symbol b indicates the control signal of the laser light, and the abscissa represents the time axis. When the laser light is scanned at a constant speed V.sub.O, distance L becomes EQU L=V.sub.O T (3),
and L and T are equivalent to each other. The symbol c indicates the distribution of the amount of exposure, which is the superposition integral of a and b, the process of which is concretely described in, for example, Japanese Patent Public Disclosure (Kokai) No. 62-104268. FIG. 7(A) is a case in which the semiconductor laser is set at the maximum light output, and turning-on for one picture element and turning-off for one picture element are repeated. The shape of the distribution c of the amount of exposure differs according to the beam diameter of the laser light, but the on-portion and the off-portion are always symmetrical to each other and the duty ratio is 1:1. That is, the ratio of the line width of a high-density portion to that of a low-density portion of a recorded image is one. FIG. 7(B) shows a case of an analog modulation in which the light output of the semiconductor laser is decreased and turning-on for one picture element and turning-off for one picture element are repeated like in the case of FIG. 7(A), and illustrates a case in which the amount of exposure is made one fourth by making the light output about one fourth of that in the case of FIG. 7(A). Also in this case, the on-portion and the off-portion in c are symmetrical to each other and duty ratio is 1:1. That is, as can be understood from the example in FIG. 7(B), when the light output is changed keeping the duty ratio of the on-off control of the semiconductor laser at 1:1, the duty ratio of the distribution of the amount of exposure is also kept at 1:1. This is, if contrast of a recorded image is changed, the ratio of the line width of a high-density portion to that of a low-density portion is kept at one. This is a desirable characteristic. However, as shown in FIG. 7(C), in the case of a pulse-width modulation in which the amount of exposure is made one fourth by making the turned-on time of the laser one fourth like b with leaving the light output of the semiconductor laser at the maximum value like a, the on-portion and the off-portion become unsymmetrical as shown by c, and the on-portion becomes shorter and the off-portion becomes longer. Accordingly, if contrast is decreased through pulse with modulation control by this conventional example for a silver-salt negative film, there occurs the disadvantage that the width of the on-portion becomes narrower and black lines become thinner. Since pulse-width modulation is performed at a high light output in the conventional cases, this phenomenon manifests itself quite clearly. Hence, it has been impossible to obtain an excellent image.
As explained above, in the conventional example in which the total emission area of the LED emission and the laser emission of a semiconductor laser is corrected, there is the disadvantage that it is actually difficult to correct its extremely nonlinear characteristic. In another conventional example in which a multivalued pulse-width modulation is performed in the laser emission area, there is the disadvantage that variations in the recorded line width are manifested due to the contrast of a recorded image, and give a bad influence on the quality of a picture.